Use of thinnings and other low specific gravity wood for lyocell pulps method

ABSTRACT

The use of low specific gravity wood from thinning operations, for example, will produce a lower brownstock viscosity for a given kappa number target. A differential of 200-cP falling ball pulp viscosity has been detected from Kraft cooks of low and high specific gravity wood. Using low specific gravity wood can reduce the bleach stage temperature and the chemical dose needed in the bleach plant to produce lyocell pulp specifications. Low specific gravity wood also increases the ability to reduce pulp viscosity to very low levels without increasing the copper number of the pulp or the concentration of carbonyl in the pulp above acceptable levels.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/842,274 filed Apr. 24, 2001, now U.S. Pat. No. 6,605,350, which is acontinuation-in-part of application Ser. No. 09/574,538, filed May 18,2000, now U.S. Pat. No. 6,331,354, which is a continuation-in-part ofapplication Ser. No. 09/256,197, filed Feb. 24, 1999, now U.S. Pat. No.6,210,801. All the above applications are herein fully incorporated byreference.

FIELD OF THE INVENTION

The present invention is directed to pulps useful for makinglyocell-molded bodies, including films, fibers, and non-woven webs, andto methods of making such pulps useful for making the lyocell-moldedbodies, to the lyocell-molded bodies made from the pulps and to themethods for making the lyocell-molded bodies. In particular, the presentinvention is directed to using “young” wood (often characterized as“core wood”, “juvenile wood”, “low specific gravity wood” or, in somecases as “thinnings”.).

BACKGROUND OF THE INVENTION

Cellulose is a polymer of D-glucose and is a structural component ofplant cell walls. These cells are referred to as fibers. Cellulosicfibers are especially abundant in tree trunks from which they areextracted, converted into pulp, and thereafter utilized to manufacture avariety of products.

Rayon is the name given to a fibrous form of regenerated cellulose thatis extensively used in the textile industry to manufacture articles ofclothing. For over a century, strong fibers of rayon have been producedby the viscose and cuprammonium processes. The latter process was firstpatented in 1890 and the viscose process two years later. In the viscoseprocess cellulose is first steeped in a mercerizing strength causticsoda solution to form an alkali cellulose. The cellulose is then reactedwith carbon disulfide to form cellulose xanthate, which is thendissolved in dilute caustic soda solution. After filtration anddeaeration, the xanthate solution is extruded from submerged spinneretsinto a regenerating bath of sulfuric acid, sodium sulfate, and zincsulfate to form continuous filaments. The resulting viscose rayon ispresently used in textiles and was formerly widely used for reinforcingrubber articles such as tires and drive belts.

Cellulose is also soluble in a solution of ammonia copper oxide. Thisproperty forms the basis for production of cuprammonium rayon. Thecellulose solution is forced through submerged spinnerets into asolution of 5% caustic soda or dilute sulfuric acid to form the fibers,which are then decoppered and washed. Cuprammonium rayon is available infibers of very low deniers and is used almost exclusively in textiles.

The foregoing processes for preparing rayon both require that thecellulose be chemically derivatized or complexed in order to render itsoluble and therefore capable of being spun into fibers. In the viscoseprocess, the cellulose is derivatized, while in the cuprammonium rayonprocess, the cellulose is complexed. In either process, the derivatizedor complexed cellulose must be regenerated and the reagents used tosolubilize it must be removed. The derivatization and regeneration stepsin the production of rayon significantly add to the cost of this form ofcellulose fiber. Consequently, in recent years attempts have been madeto identify solvents that are capable of dissolving underivatizedcellulose to form a dope of underivatized cellulose from which fiberscan be spun.

One class of organic solvents useful for dissolving cellulose are theamine N-oxides, in particular the tertiary amine N-oxides. For example,Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxidematerials suitable as solvents. Johnson, in U.S. Pat. No. 3,447,939,describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) andother amine N-oxides as solvents for cellulose and many other naturaland synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and4,196,282, deal with the difficulties of dissolving cellulose in amineoxide solvents and of achieving higher concentrations of cellulose.

Lyocell is an accepted generic term for a cellulose fiber precipitatedfrom an organic solution in which no substitution of hydroxyl groupstakes place and no chemical intermediates are formed. Severalmanufacturers presently produce lyocell fibers, principally for use inthe textile industry. For example, Acordis, Ltd. presently manufacturesand sells a lyocell fiber called Tencel® fiber.

Currently available lyocell fibers are produced from wood pulps thathave been extensively processed to remove non-cellulose components,especially hemicellulose. These highly processed pulps are referred toas dissolving grade or high alpha (or high α) pulps, where the termalpha (or α) refers to the percentage of cellulose. Thus, a high alphapulp contains a high percentage of cellulose, and a correspondingly lowpercentage of other components, especially hemicellulose. The processingrequired to generate a high alpha pulp significantly adds to the cost oflyocell fibers and products manufactured therefrom.

Since the conventional Kraft process stabilizes residual hemicellulosesagainst further alkaline attack, it is not possible to obtain acceptablehigh alpha pulps for lyocell products, through subsequent treatment ofKraft pulp in the conventional bleaching stages. In order to preparehigh alpha pulps by the Kraft process, it is necessary to pretreat thewood chips in an acid phase before the alkaline pulping stage. Asignificant amount of material, primarily hemicellulose, on the order of10% or greater of the original wood substance, is solubilized in thisacid phase pretreatment and thus process yields drop. Under theseconditions, the cellulose is largely resistant to attack, but theresidual hemicelluloses are degraded to a much shorter chain length andare therefore removed to a large extent in the subsequent Kraft cook bya variety of hemicellulose hydrolysis reactions or by dissolution. Thedisadvantage of conventional high alpha pulps used for lyocell is theresulting loss of yield by having to eliminate hemicelluloses.

In view of the expense of producing commercial high alpha pulps, itwould be desirable to have alternatives to conventional high alpha pulpsfor making lyocell products. In addition, manufacturers would like tominimize the capital investment necessary to produce such types of pulpsby utilizing existing capital plants. Thus, there is a need forrelatively inexpensive, low alpha (e.g., high yield, high hemicellulose)pulps that have attributes that render them useful in lyocell-moldedbody production.

In U.S. Pat. No. 6,210,801, fully incorporated herein by reference inits entirety, assigned to the assignee of the present application, lowviscosity, high hemicellulose pulp is disclosed that is useful forlyocell-molded body production. The pulp is made by reducing theviscosity of the cellulose without substantially reducing thehemicellulose content. Such processes use an acid, or an acidsubstitute, or other methods therein described.

While the methods described in the '801 patent are effective at reducingthe average degree of polymerization (D.P.) of cellulose withoutsubstantially decreasing the hemicellulose content, a further needexisted for a process that did not require a separate copper numberreducing step and which was readily adaptable to pulp mills that haveoxygen reactors, multiple alkaline stages and/or alkaline conditionssuitable for substantial D.P. reduction of bleached or semi-bleachedpulp. Environmental concerns have also generated a great interest inusing bleaching agents that reduce the use of chlorine compounds. Inrecent years, the use of oxygen as a delignifying agent has occurred ona commercial scale. Examples of equipment and apparatus useful forcarrying out an oxygen stage delignification process are described inU.S. Pat. Nos. 4,295,927; 4,295,925; 4,298,426; and 4,295,926. In U.S.Pat. No. 6,331,554, assigned to the assignee of the present application,fully incorporated herein by reference in their entirety, a highhemicellulose, low viscosity pulp is disclosed that is useful forlyocell-molded body formation. The pulp is made from an alkaline pulp bytreating the alkaline pulp with an oxidizing agent in a medium to highconsistency reactor to reduce the D.P. of the cellulose, withoutsubstantially reducing the hemicellulose or increasing the coppernumber.

Further efforts to reduce the cost of making lyocell-molded bodies hasresulted in U.S. application Ser. No. 09/842,274, now U.S. Pat. No.6,605,350, fully incorporated by reference in its entirety. In the '274application, the assignee of the present invention describes pulps madefrom sawdust and other low fiber length wood using a procedure similarto that of the '554 patent. These pulps are high in hemicellulose andlow in viscosity, which makes them especially suitable forlyocell-molded body formation.

The forest industry continues to generate vast quantities of byproductsin the normal course of day-to-day forestry management and woodprocessing. These byproducts are for the most part underutilized. Theneed to conserve resources by utilizing wood byproducts in new wayspresents a unique opportunity. It would be advantageous to develop a lowcost pulp that is useful for making lyocell-molded bodies from all thisunderutilized wood, namely from the core wood or young or juvenile woodsuch as thinnings, hereafter referred to as low specific gravity wood.Thus, presenting a low cost alternative to the highly refined high-alphapulps.

SUMMARY OF THE INVENTION

One embodiment of the invention is a pulp having at least 7% by weighthemicellulose; a viscosity of less than or about 32 cP; a copper numberless than or about 2; a weighted average fiber length less than or about2.7 mm; and a coarseness less than or about 23 mg/100 m. In anotherembodiment of the invention, a method for making lyocell-molded body isprovided. The method includes dissolving a pulp in a solvent to form acellulose solution; forming a lyocell-molded body from the solution; andregenerating the molded body, wherein the pulp has at least 7% by weighthemicellulose, a viscosity less than or about 32 cP; a copper numberless than or about 2; a weighted average fiber length less than or about2.7 mm; and a coarseness less than or about 23 mg/100 m. The method canuse a meltblowing, centrifugal spinning, spun bonding, or dry-jet wettechnique.

In another embodiment of the invention, a method of making a pulp isprovided. The method includes pulping of wet material with a specificgravity less than or about 0.41 using an alkaline pulping process; andbleaching the pulp to reduce the viscosity of the pulp to or about 32 cPor lower. The bleached pulp has at least 7% hemicellulose by weight, acopper number less than or about 2, a weighted average fiber length lessthan or about 2.7 mm, and a coarseness less than or about 23 mg/100 m.

In another embodiment of the invention, a lyocell product is provided.The lyocell product has at least 7% hemicellulose by weight, andcellulose, wherein the pulp used to make the product has a viscosityless than or about 32 cP, a copper number less than or about 2, aweighted average fiber length less than or about 2.7 mm, and acoarseness less than or about 23 mg/100 m. Lyocell products can befibers, films, or non-woven webs, for example.

The use of low specific gravity wood can produce a lower brownstockviscosity for a given kappa number target. Using wood with low specificgravity values reduce the bleach stage temperature and the chemical doseneeded in the bleach plant to produce pulp having acceptable lyocellspecifications. Low specific gravity wood results in very low viscositylevels without increasing the copper number of the pulp or theconcentration of carbonyl in the pulp above acceptable levels. Theprocess does not use an acid phase pretreatment prior to pulping, andthe subsequent bleaching conditions do not result in a substantialdecrease in hemicellulose content.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a flowsheet illustrating one embodiment of a method of makinga pulp according to the present invention; and

FIG. 2 is a flow sheet illustrating one embodiment of a method of makinga lyocell-molded body according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a suitable method to produce a lyocelldissolving pulp from low specific gravity wood is illustrated. Themethod may be considered to include two broad processing areas, pulpingdepicted as block 126 and bleaching depicted as block 128.

In block 100, low specific gravity wood chips are loaded or fed into adigester. Specific gravity, according to The Handbook of Pulping andPapermaking, 2d ed., by Christopher J. Biermann, is the (unit less)ratio of the solid wood density to the density of water at the sametemperature. As used herein, specific gravity is the average specificgravity of any population of wood feedstock material. The solid wooddensity may be determined using the green volume, the oven-dry volume,or intermediate volumes. The wood chips used in practicing the inventioncan be made from any cellulose source. Contrary to conventionalthinking, low specific gravity wood has been found to be suitable foruse as a source of cellulose for making lyocell-molded bodies. Asuitable range of low specific gravity wood used for the presentinvention is any wood material having a specific gravity about equal orless than 0.41. Low specific gravity wood results in a lower brownstockpulp viscosity, which is believed to reduce the use of bleachingchemicals in the bleach plant. Representative sources of low specificgravity wood may be derived from “thinnings” and “juvenile” wood.Juvenile wood is defined as the first 10 growth rings surrounding thepith, according to Biermann. However, others define it as wood formednear the pith of the tree, often characterized by wide growth rings,lower density, and shorter fibers. However, in some instances thejuvenile wood may extend to the 15-ring or more. Specific gravityincreases with the increasing height of the tree, so specific gravity at16 feet, 32 feet, or 48 feet is incrementally greater than at the buttof the tree. In some embodiments, the specific gravity will be less than0.41, and could be less than 0.38, 0.36, 0.34, 0.32, or 0.30, or less.

Digesters for use in the present invention can include any digestersuitable to pulp low specific gravity wood. One example of a suitabledigester is a continuous digester that is often referred to as a “Kamyr”digester. (It should be noted that Kamyr is the name of a Company thatdesigned and built such digesters and as such, the term Kamyr is looselyassociated with a continuous digester. Kamyr no longer exists as aCompany. Such continuous digesters are supplied by Kvaerner.) Thesedigesters have been used in the pulp and paper industry for severalyears with the first one being installed in Sweden in 1950. Over theyears, the modifications have been made to these digesters to improvetheir operation. The digester system may be either a single vessel or atwo-vessel system.

“Kamyr” digesters are typically used in Kraft or alkaline wood pulping,but may also be used in semi-chemical pulping methods. Other continuousdigesters, such as the M&D digester and the Pandia digester, are alsosuitable to use in the present invention. However, the present inventioncan also be practiced using any batch or other continuous digester.

Referring to FIG. 1, within the pulping process, block 126, there areseveral operations, depicted as blocks 100-116. Loading, or feedingchips as discussed above, occurs in block 100. The wood chips may bepresteamed prior to cooking, block 102. Steam at atmospheric pressurepreheats the chips and drives off air so that liquor penetration will beenhanced. After the pre-steaming operation is completed, cooking liquor,referred to as white liquor, containing the pulping chemicals may beadded to the chips, block 104. The white liquor and chips are then fedinto the digester. In Kraft pulping, the active chemical compounds areNaOH and Na₂S. Other chemicals may be added to influence or impartdesirable effects on the pulping process. These additional chemicals arewell known to those of skill in the art. The present invention providesa lower brownstock pulp viscosity from relatively lower specific gravitywood as composed with wood having a higher specific gravity, i.e.,specific gravity is related to Kappa number.

Impregnation, block 106, is the period during which the chemicals areallowed to impregnate the low specific gravity wood material. Goodliquor penetration helps assure a uniform cooking of the chips.

“Cooking” occurs in blocks 108 and 110. The co-current liquid contactoperation, block 108, is followed by the counter-current liquid contactoperation, block 110. Cooking of the low specific gravity wood occursduring these two operations. In either block 108 or 110, the cookingliquor and chips can be brought to temperature.

Digester washing, block 112, is accomplished by introducing wash liquorinto the bottom of the digester and having it flow counter-current tothe cooked pulp. Cooking for the most part ends when the pulp encountersthe cooler wash liquor.

Upon completion of the cook operation, and digester washing, thedigester contents are blown, block 112. Digester blowing involvesreleasing the wood chips and liquor at atmospheric pressure. The releaseoccurs with a sufficient amount of force to cause fiber separation. Ifdesired, the blow tank may be equipped with heat recovery equipment toreduce operating expenses.

In block 114, the pulp is sent from the blow tank to external brownstockpulp washers. The separation of black liquor from the pulp occurs at thebrownstock washers.

In one embodiment of a method of making a pulp from low specific gravitywood to be used in the manufacture of lyocell-molded bodies, the timeallowed for impregnation in block 106 is about 35 minutes. The initialpercent effective alkali is about 8.5. The percent effective alkali at 5minutes is about 1.6. The percent sulfidity is about 29. The liquorratio is about 4. The initial temperature is about 110° C. The residualgrams per liter of effective alkali is about 9.63. The residual percenteffective alkali is about 3.85. The pH is about 12.77, and the H factoris about 2.

In one embodiment of the co-current operation, block 108, the percenteffective alkali is about 4.2. According to Biermann, the effectivealkali is the ingredients that will actually produce alkali underpulping conditions. The percent sulfidity is about 29. According toBiermann, the sulfidity is the ratio of sodium sulfide to the activealkali, expressed as a percent. The liquor addition time is about 1minute. The temperatures may be ramped to the final cooking temperaturewith a hold at one or more temperatures. The first temperature platformis about 154° C. The time to reach the temperature is about 9 minutesand the time at the temperature is about 5 minutes. A second and highercooking temperature at the co-current operation is provided at 170° C.The time to reach the second temperature is about 51 minutes and thetime at temperature is about 3 minutes. The effective alkali remainingafter a cook operation is called the residual alkali. The residual gramsper liter of effective alkali is about 9.42, following the co-currentoperation. The residual percent effective alkali is about 3.77. The pHis about 12.92, and the H factor is about 649.

In one embodiment of the counter-current operation, block 110, thepercent effective alkali is about 8. The percent sulfidity is about29.2. Capability also exists for ramping to two different temperaturesin the counter-current step. However, in one embodiment, the first andsecond cooking temperatures are both about 171° C. The time to reachtemperature is about 54 minutes and the time at the temperature is about162 minutes. The effective alkali grams per liter is about 16.0. Thedisplacement rate is about 93 ml per minute. The displacement volume isabout 20 liters. The volumes given here are relatively small, since themethod was tested on a lab-scale bench reactor. However, with theparameters provided herein, and with no undue experimentation, theprocess can be scaled to any rate. The residual grams per liter ofeffective alkali is about 9.95. The residual percent effective alkali isabout 3.98. The pH is about 12.74 and the H factor is about 3877. In oneembodiment, the total time is about 319 minutes and the percenteffective alkali for the total cook is about 22.3.

In one embodiment, after washing, the viscosity of the brownstock pulpis about 153 cP. The total yield on oven dried wood is about 41.04.

Following the pulping process, generally depicted as reference numeral126 in FIG. 1, the brownstock pulp made from low specific gravity woodis bleached to reduce its viscosity. The bleaching process does not leadto a substantial reduction of the hemicellulose content of the pulp. Themethod according to the invention produces a bleached dissolving pulpthat is suitable for lyocell-molded body production. Bleaching ofchemical pulps involves the removal of lignin with an attendant decreasein the pulp fiber length and viscosity. However, the bleaching processdoes not cause a substantial reduction to the hemicellulose content ofthe pulp. Bleaching brownstock pulp made from low specific gravity woodmay require fewer chemicals than the conventional highly refined,high-alpha pulps presently being used for lyocell.

In one embodiment, the low specific gravity brownstock pulp madeaccording to the invention can be treated with various chemicals atdifferent stages in the bleach plant. The stages are carried out invessels or towers of conventional design. One representative bleachingsequence is ODE_(P)D. The operations occurring in the bleaching plantare represented collectively by reference numeral 128 in FIG. 1. Otherembodiments of post bleaching the pulp after pulping are described inU.S. Pat. No. 6,331,354, and U.S. application Ser. No. 09/842,274,incorporated herein by reference in their entirety.

The first stage of bleaching is an O stage, block 116. The O stagecomprises bleaching with oxygen. However, according to Biermann, someconsider oxygen bleaching to be an extension of the pulping process.Oxygen bleaching is the delignification of pulps using oxygen underpressure. The oxygen is considered to be less specific for the removalof lignin than the chlorine compounds. Oxygen bleaching takes place inan oxygen reactor. Suitable oxygen reactors capable of carrying out themethod of the present invention are described in U.S. Pat. Nos.4,295,925; 4,295,926; 4,298,426; and 4,295,927, fully incorporatedherein by reference in their entirety. The reactor can operate at a highconsistency, wherein the consistency of the feedstream to the reactor isgreater than 20% or it can operate at medium consistency, where themedium consistency ranges between 8% up to 20%. Preferably, if a highconsistency oxygen reactor is used, the oxygen pressure can reach themaximum pressure rating for the reactor, but more preferably is greaterthan 0 to about 85 psig. In medium consistency reactors, the oxygen canbe present in an amount ranging from greater than 0 to about 100 poundsper ton of the pulp, but is more preferably about 50 to about 80 poundsper ton of pulp. The temperature of the O stage ranges from about 100°C. to about 140° C.

In one embodiment of the method to make a pulp suitable to be used inmaking lyocell-molded bodies, a D stage, block 118 follows the O stage,block 116. The D stage comprises bleaching the pulp coming from theoxygen reactor with chlorine dioxide. Chlorine dioxide is more selectivethan oxygen for removing lignin. The amount of chlorine dioxide used inthis stage ranges from about 20 to about 30 lb/ton, which may be lowerthan a conventional bleach plant that processes pulp from wood chipswith a specific gravity not within the low specific gravity range ofthis invention. The temperature of the D stage ranges from about 50° C.to about 85° C.

In one embodiment of the method to make a pulp suitable to be used inmaking lyocell-molded bodies, an E_(p) stage, block 120, follows the Dstage, block 118. The E_(p) stage is the hydrogen peroxide reinforcedextraction stage where lignin is removed from the pulp using caustic inan amount ranging from about 20 to about 50 lb/ton. The amount ofhydrogen peroxide ranges from about 20 to about 60 lb/ton, which may belower than a conventional bleach plant that processes pulp from woodchips having a specific gravity not considered within the low specificgravity range of this invention. The temperature of the E_(p) stageranges from about 75 to about 95° C.

In one embodiment, a second D stage, block 122, follows the E_(p) stage,block 120. The amount of chlorine dioxide used in this stage ranges from10 to about 30 lb/ton, which may be lower than a conventional bleachplant that processes pulp from wood chips having a conventional specificgravity not considered to be within the low specific gravity range ofthis invention. The temperature of the D stage ranges from about 60° C.to about 90° C.

One embodiment of a pulp made from low specific gravity wood has ahemicellulose content of at least 7% hemicellulose, a pulp viscosityless than 32 cP, a copper number less than 2.0, and in some instancesless than 1.3 (TAPPI T430), a weighted average fiber length less than2.7 mm, and a coarseness less than 23 mg/100 m. Other embodiments ofpulps made according to the present invention have a combined copper,manganese, and iron content less than 2 ppm, a total metal load lessthan 300 ppm, and a silicon content less than 50 ppm. Lyocell moldedbodies made from the pulps of the invention will have a correspondinglyhigh hemicellulose content of at least 7% by weight, and cellulose.

Hemicellulose is measured by a sugar content assay based on TAPPIstandard T249 hm-85.

Methods for measuring pulp viscosity are well known in the art, such asTAPPI T230. Copper number is a measure of the carboxyl content of pulp.The copper number is an empirical test used to measure the reducingvalue of cellulose. The copper number is expressed in terms of thenumber of milligrams of metallic copper, which is reduced from cuprichydroxide to cuprous oxide in an alkaline medium by a specified weightof cellulosic material. The degree to which the copper number changesduring the bleaching operation is determined by comparing the coppernumber of the brownstock pulp entering the bleaching plant and thecopper number of the bleached pulp after the bleaching plant. A lowcopper number is desirable because it is generally believed that a highcopper number causes cellulose and solvent degradation during and afterdissolution of the bleached pulp to form a dope.

The weighted average fiber length (WAFL) is suitably measured by a FQAmachine, model No. LDA93-R9704, with software version 2.0, made by theOptest Company of Hawkesbury, Ontario, Canada.

Coarseness is measured using Weyerhaeuser Standard Method WM W-FQA.

Transition metals are undesirable in pulp because they accelerate thedegradation of cellulose and NMMO in the lyocell process. Examples oftransition metals commonly found in bleached pulps include iron, copper,and manganese. Preferably, the combined metal content of these threemetals in the pulps of the invention is less than about 20 ppm byWeyerhaeuser Test No. AM5-PULP-1/6010.

Additionally, pulps of the invention have a total metal load of lessthan 300 ppm by Weyerhaeuser Test No. AM5-PULP-1/6010. The total metalload refers to the combined amount, expressed in units of parts permillion (ppm), of nickel, chromium, manganese, iron and copper.

Once the pulp has been bleached to reduce its viscosity withoutsubstantially increasing its copper number or decreasing thehemicellulose content, the pulp can either be washed in water andtransferred to a bath of organic solvent, such asN-methyl-morpholine-N-oxide (NMMO), for dissolution prior tolyocell-molded body formation. Alternatively, the bleached washed pulpcan be dried and broken into fragments for storage and/or shipping in aroll, sheet or bale, for example.

In order to make lyocell products from the low specific gravity woodpulps, the pulp is first dissolved in an amine oxide, preferably atertiary amine oxide. Representative examples of amine oxide solventsuseful in the practice of the present invention are set forth in U.S.Pat. No. 5,409,532, incorporated herein by reference in its entirety.The preferred amine oxide solvent is NMMO. Other representative examplesof solvents useful in the practice of the present invention includedimethylsulfoxide (D.M.S.O.), dimethylacetamide (D.M.A.C.),dimethylformamide (D.M.F.) and caprolactan derivatives. The bleachedpulp is dissolved in amine oxide solvent by any known means such as onesset forth in U.S. Pat. Nos. 5,534,113; 5,330,567; and 4,246,221,incorporated herein by reference in their entirety. The pulp solution iscalled dope. The dope is used to manufacture lyocell fibers, films, andnonwovens or other products, by a variety of techniques, including meltblowing, spunbonding, centrifugal spinning, dry-jet wet, or any othersuitable method. Examples of some of these techniques are described inU.S. Pat. Nos. 6,235,392; 6,306,334; 6,210,802; and 6,331,354,incorporated herein by reference in their entirety. Examples oftechniques for making films are set forth in U.S. Pat. Nos. 5,401,447;and 5,277,857, incorporated herein by reference in their entirety.Meltblowing, centrifugal spinning and spunbonding used to make lyocellfibers and nonwoven webs are described in U.S. Pat. Nos. 6,235,392 and6,306,334, incorporated herein by reference in their entirety. Dry-jetwet techniques are more fully described in U.S. Pat. Nos. 6,235,392;6,306,334; 6,210,802; 6,331,354; and 4,142,913; 4,144,080; 4,211,574;4,246,221; incorporated herein by reference in their entirety.

One embodiment of a method for making lyocell products, includingfibers, films, and nonwoven webs from dope derived from pulp isprovided, wherein the pulp is made from low specific gravity wood, thepulp having at least 7% hemicellulose, a viscosity less than or about 32cP, a copper number less than or about 2, a weighted average fiberlength less than or about 2.7 mm, and a coarseness less than or about 23mg/100 m. The method involves extruding the dope through a die to form aplurality of filaments, washing the filaments to remove the solvent,regenerating the filaments with a nonsolvent, including water oralcohol, and drying the filaments.

FIG. 2 shows a block diagram of one embodiment of a method for forminglyocell fibers from the pulps made from low specific gravity woodaccording to the present invention. Starting with low specific gravitywood pulp in block 200, the pulp is physically broken down, for exampleby a shredder in block 202. The pulp is dissolved with an amineoxide-water mixture to form a dope, block 204. The pulp can be wettedwith a nonsolvent mixture of about 40% NMMO and 60% water. The mixturecan be mixed in a double arm sigma blade mixer and sufficient waterdistilled off to leave about 12-14% based on NMMO so that a cellulosesolution is formed, block 208. Alternatively, NMMO of appropriate watercontent may be used initially to eliminate the need for the vacuumdistillation block 208. This is a convenient way to prepare spinningdopes in the laboratory where commercially available NMMO of about40-60% concentration can be mixed with laboratory reagent NMMO havingonly about 3% water to produce a cellulose solvent having 7-15% water.Moisture normally present in the pulp should be accounted for inadjusting the water present in the solvent. Reference is made toarticles by Chanzy, H., and A. Peguy, Journal of Polymer Science,Polymer Physics Ed. 18:1137-1144 (1980), and Navard, P., and J. M.Haudin, British Polymer Journal, p. 174 (December 1980) for laboratorypreparation of cellulose dopes in NMMO and water solvents.

The dissolved, bleached pulp (now called the dope) is forced throughextrusion orifices in a process called spinning, block 210, to producecellulose filaments that are then regenerated with a non-solvent, block202. Spinning to form lyocell-molded bodies, including fibers, films,and nonwovens, may involve meltblowing, centrifugal spinning, spunbonding, and dry-jet wet techniques. Finally, the lyocell filaments orfibers are washed, block 214.

The solvent can either be disposed of or reused. Due to its high costs,it is generally undesirable to dispose of the solvent. Regeneration ofthe solvent suffers from the drawback that the regeneration processinvolves dangerous, potentially explosive conditions.

The following examples merely illustrate the best mode now contemplatedfor practicing the invention, but should not be construed to limit theinvention.

EXAMPLE 1

A commercial continuous extended delignification process was simulatedin the laboratory utilizing a specially built reactor vessel withassociated auxiliary equipment, including circulating pumps,accumulators, and direct heat exchangers, etc. Reactor temperatures werecontrolled by indirect heating and continuous circulation of cookingliquor. The reactor vessel was charged with a standard quantity ofequivalent moisture free wood. An optional atmospheric pre-steaming stepmay be carried out prior to cooking. A quantity of cooking liquor,ranging from about 50% to 80% of the total, was then charged to thedigester along with dilution water to achieve the target liquor to woodratio. The reactor was then brought to impregnation temperature andpressure and allowed to remain for the target time. Following theimpregnation period, an additional portion of the total cooking liquorwas added to the reactor vessel, ranging from about 5% to 15% of thetotal. The reactor was then brought to cooking temperature and allowedto remain there for the target time period to simulate the co-currentportion of the cook.

Following the co-current portion of the cook, the remainder of thecooking liquor was added to the reactor vessel at a fixed rate. The rateis dependent on the target time period and proportion of cooking liquorused for this step of the cook. The reactor was controlled at a targetcooking temperature and allowed to remain there during the simulation ofthe counter-current portion of the cook. Spent cooking liquor waswithdrawn from the reactor into an external collection container at thesame fixed rate. At the end of the cook, the reactor vessel was slowlydepressurized and allowed to cool below the flash point. The reactorvessel was opened and the cooked wood chips were collected, drained ofliquor, washed, screened and made ready for testing. Three cooks of lowspecific gravity wood chips were prepared, along with three cooks ofnon-low specific gravity wood.

EXAMPLE 2 PULPING PROCESS PARAMETERS FOR LOW SPECIFIC GRAVITY WOOD

One cook for low specific gravity wood chips had the followingparameters.

TABLE 1 Wood Chip S.G. 0.410 Pre-Steam @ 110 C., minutes 5 Impregnation:Time, minutes 35 % Effective Alkali, initial 8.5 % EA, second @ 5minutes 1.6 % sulfidity 29 Liquor ratio 4 Temperature - degrees C. 110Residual, G/L EA 9.63 Residual, % EA 3.85 PH 12.77 H-factor 2 PressureRelief Time, Minutes 3 Co-Current: % Effective Alkali 4.2 % sulfidity 29liquor addition time, minutes 1 temperature - degrees C. 154 time to,minutes 9 time at, minutes 5 temperature - degrees C. 170 time to,minutes 51 time at, minutes 3 residual, G/L EA 9.42 residual, % EA 3.77PH 12.92 H-Factor 649 Counter-Current: % effective alkali 8 % sulfidity29.2 temperature - degrees C. 171 time to, minutes 54 time at, minutes 0temperature - degrees C. 171 time to, minutes 0 time at, minutes 162 EA,G/L - strength 16.0 displacement rate, CC/M 93 displacement volume,liters 20.00 residual, G/L EA 9.95 residual, % EA 3.98 PH 12.74 H-factor3877 Total Time, Minutes 319 % Effective Alkali - Total Cook 22.3Accepts, % on O.D. Wood 41.01 Rejects, % on O.D. Wood 0.03 Total Yield,% on O.D. Wood 41.04 Kappa Number, 10 Minutes 16.80

EXAMPLE 3 BLEACHING PROCESS FOR LOW SPECIFIC GRAVITY WOOD

The pulp made by the process of Example 2 was bleached according to thefollowing procedure.

O Stage

Inwoods low specific gravity wood chips were pulped into an alkalineKraft pulp with a kappa number of 16.8 (TAPPI Standard T236 cm-85 and aviscosity of 239 cP (TAPPI T230). The brownstock pulp was treated withoxygen in a pressure vessel with high consistency mixing capabilities.The vessel was preheated to about 120° C. An amount of sodium hydroxide(NaOH) equivalent to 100 pounds per ton of pulp was added to thealkaline pulp. The reaction vessel was then closed and the pressure wasincreased to 60 psig by introducing oxygen into the pressure vessel.Water was present in the vessel in an amount sufficient to provide a 10%consistency.

After 45 minutes, the stirring was stopped and the pulp was removed fromthe pressure vessel and washed. The resulting washed pulp viscosity was35.3 cP, and had a kappa number of 3.8.

D Stage

The D stage treated the pulp processed in the O stage by washing itthree times with distilled water, pin fluffing the pulp, and thentransferring the pulp to a polypropylene bag. The consistency of thepulp in the polypropylene bag was adjusted to 10% with the addition ofwater. Chlorine dioxide corresponding to an amount equivalent to 28.4pounds per ton of pulp was introduced to the diluted pulp by dissolvingthe chlorine dioxide in the water used to adjust the consistency of thepulp in the bag. The bag was sealed and mixed and then held at 75° C.for 30 minutes in a water bath. The pulp was removed and washed withdeionized water.

E_(p) Stage

The washed pulp from the D stage was then placed in a freshpolypropylene bag and caustic was introduced with one-half of the amountof water necessary to provide a consistency of 10%. Hydrogen peroxidewas mixed with the other one-half of the dilution water and added to thebag. The hydrogen peroxide charge was equivalent to 40 pounds per ton ofpulp. The bag was sealed and mixed and held for 55 minutes at 88° C. ina water bath. After removing the pulp from the bag and washing it withwater, the mat was filtered and then placed back into the polypropylenebag and broken up by hand.

D Stage Chlorine dioxide was introduced a second time to the pulp in anamount equivalent to 19 pounds per ton of pulp with the dilution waternecessary to provide a consistency of 10%. The bag was sealed and mixed,and then held for 3 hours at 88° C. in a water bath. The treated pulphad a copper number of about 0.9 measured by TAPPI Standard T430 and hada hemicellulose (xylan and mannan) content of 12.7%.

EXAMPLE 4

Low specific gravity wood having a specific gravity of 0.410 was pulpedusing the Kraft process, and subsequently, bleached and treated withvarying amounts of oxygen to reduce its viscosity. Components in thepulps made using Inwoods low specific gravity wood chips are 7.2% xylansand 5.5% mannans.

Table 2 shows the results for three different cooking conditions. Whilebrownstock pulp WAFL is provided, it is apparent that bleaching thebrownstock pulp to reduce its viscosity without substantially reducingthe hemicellulose content, in accordance with the conditions of thepresent invention, will not result in any appreciable increase in thebleached pulp WAFL and may in fact be lower than the brownstock pulpWAFL.

TABLE 2 Inwoods Inwoods Inwoods chips chips chips Cook A Cook B Cook CChips Specific Gravity 0.410 0.410 0.410 Kappa of Brownstock 24.4 20.116.8 Yield % 43.2 41.4 41.0 Brownstock pulp viscosity 414 235 153 (cP)Falling Ball Brownstock pulp WAFL 2.70 2.70 2.69 (mm) Brownstock pulpCoarseness 18.3 17.9 17.6 (mg/100 m) O2 pulp viscosity cP 55 34 28 (100lbs/ton NaOH) 7.6 kappa 6.0 kappa 3.8 kappa O2 pulp viscosity cP 80 6349 (60 lbs/ton NaOH) 6.0 kappa 7.5 kappa 5.6 kappa Bleached pulpcoarseness 32.4 21.8 (mg/100 m) Bleached pulp fibers/g × 10⁶ 4.8 4.6Bleached pulp viscosity (cP) 31.8 29.5 Bleached pulp intrinsic 4.1 4.2viscosity Bleached pulp Cu (ppm) 0.6 <0.6 Bleached pulp Fe (ppm) 12 14.3Bleached pulp Mn (ppm) 1.5 3.6 Bleached pulp Cr (ppm) <0.4 <0.3 Bleachedpulp Si (ppm) 41 31

COMPARATIVE EXAMPLE 5

Pulping Process Parameters for Non-Low Specific Gravity Wood

A conventional Tolleson wood chip made from wood having specific gravityof 0.495 was pulped using a Kraft process and subsequently treated withvarying amounts of oxygen to reduce its viscosity. Table 3 shows thepulping conditions for one cook of Tolleson wood chips.

TABLE 3 Wood Chips S.G. 0.495 Pre-Steam @ 110 C., minutes 5Impregnation: time, minutes 35 % Effective Alkali, initial 8.5 % EA,second @ 5 minutes 1.6 % sulfidity 30.5 liquor ratio 4 temperature -degrees C. 110 residual, G/L EA 9.17 residual, % EA 3.67 PH 13.24H-factor 2 Pressure Relief Time, Minutes 2 Co-Current: % EffectiveAlkali 4.2 % sulfidity 30.5 liquor addition time, minutes 1temperature - degrees C. 157 time to, minutes 14 time at, minutes 0temperature - degrees C. 170 time to, minutes 54 time at, minutes 0residual, G/L EA 8.31 residual, % EA 3.32 PH 13.07 H-Factor 680Counter-Current: % Effective Alkali 8 % sulfidity 30.0 Temperature -degrees C. 171 Time to, minutes 54 Time at, minutes 0 Temperature -degrees C. 171 Time to, minutes 0 Time at, minutes 162 EA, G/L -strength 20.4 Displacement rate, CC/M 73 Displacement volume, liters15.87 Residual, G/L EA 9.72 residual, % EA 3.89 PH 13.18 H-factor 3975Total Time, Minutes 319 % Effective Alkali - Total Cook 22.3 Accepts, %on O.D. Wood 44.23 Rejects, % on O.D. Wood 0.13 Total Yield, % on O.D.Wood 44.36 Kappa Number, 10 Minutes 17.75

Table 4 shows the results of three different cooks using a conventionalTolleson wood chip made from a non-low specific gravity wood. Componentsin the pulps made using Tolleson non-low specific gravity wood chips are6.5% xylose; 6.6% mannose; 5.7% xylans; and 5.9% mannans.

TABLE 4 Tolleson Tolleson Tolleson chips chips chips Cook A Cook B CookC Chips Specific Gravity 0.495 0.495 0.495 Kappa of Brownstock 26.9 20.817.8 Yield % 46.6 46.1 44.4 Brownstock pulp 633 358 243 viscosity (cP)Falling Ball Brownstock pulp WAFL 4.13 4.14 4.19 (mm) Brownstock pulp26.1 24.4 24.3 Coarseness (mg/100 m) O2 pulp viscosity cP 96 43 41 (100lbs/ton NaOH) 6.4 6.9 4.7 kappa kappa kappa O2 pulp viscosity cP 180 8870 (60 lbs/ton NaOH) 8.3 5.5 6.2 kappa kappa kappa Bleached pulpcoarseness 24.9 27.5 (mg/100 m) Bleached pulp 3.8 2.8 fibers/g × 10⁶Bleached pulp viscosity 28.5 24.2 (cP) Bleached pulp intrinsic 4.3 4viscosity Bleached pulp Cu (ppm) <0.6 <0.7 Bleached pulp Fe (ppm) 11.516 Bleached pulp Mn (ppm) 5 6 Bleached pulp Cr (ppm) <0.4 0.3 Bleachedpulp Si (ppm) ≧1 32

It can be seen that the viscosity of the pulps made from the Inwoods lowspecific gravity wood chips is lower than the viscosity of the pulpsmade from the Tolleson non-low specific gravity wood chips.

It can be seen that the viscosity of the pulps made from the Inwoods lowspecific gravity wood chips is lower than the viscosity of the pulpsmade from the Tolleson non-low specific gravity wood chips.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of making apulp, comprising: pulping a wood material with a specific gravity lessthan 0.41 by an alkaline pulping process; and bleaching the pulp toprovide a bleached pulp with at least 2% by weight hemicellulose andweighted average fiber length less than 2.7 mm.
 2. The method of claim1, wherein the viscosity is 32 cP or lower.
 3. The method of claim 1,wherein said bleached pulp has a copper number less than
 2. 4. Themethod of claim 1, wherein said bleached pulp has a coarseness less than23 mg/100 m.